Rheumatoid nodule masquerading as a bone metastasis: a diagnostic case report

Rheumatoid arthritis (RA) presents with extra-articular manifestations in 15-25% of cases. Intra-osseous rheumatoid nodules occur in <1% of patients and mimic metastatic disease on imaging. When extra-articular features precede joint symptoms, diagnostic delays are common.

A 78-year-old male smoker presented with chronic cough and dyspnea. CT revealed interstitial lung disease and a pulmonary nodule. PET-CT demonstrated multiple FDG-avid osteolytic lesions in ribs, scapula, and spine (SUV 4.2-8.7), suggesting metastatic malignancy. Initial biopsies showed inflammation without malignancy. Four months later, the patient developed polyarthralgia, morning stiffness, and 8-kg weight loss. Hand radiographs revealed erosive arthropathy. Laboratory tests showed rheumatoid factor 240 IU/mL, anti-CCP 185 U/mL, ESR 68 mm/hr, and CRP 42 mg/L. Repeat scapular biopsy identified necrobiotic granulomas consistent with rheumatoid nodules.

Intra-osseous rheumatoid nodules result from immune complex-mediated tissue necrosis. Inflammatory cytokines activate osteoclasts, creating lytic lesions with high metabolic activity indistinguishable from metastases on PET imaging. Extra-articular manifestations precede joint symptoms in 10-15% of RA cases. Anchoring bias toward malignancy in an elderly smoker with pulmonary nodules and osteolytic lesions delayed alternative diagnoses. Negative initial biopsies and subsequent development of erosive arthropathy prompted diagnostic reconsideration.

Extra-articular RA can masquerade as metastatic disease. Intra-osseous rheumatoid nodules represent rare manifestations requiring histological confirmation. Clinicians should consider systemic autoimmune diseases when evaluating unexplained interstitial lung disease with multiple osteolytic lesions, particularly when malignancy screening is negative. Early recognition enables appropriate immunosuppressive therapy and prevents irreversible organ damage.